The big mystery of the unemployment data is why, on some measures, things do not look so bad.

Yesterday the so-called "claimant count" of those seeking unemployment benefit showed only a small rise, while the broader Labour Force Survey (LFS) measure revealed a far bigger jump.

The divergence between the two has grown in recent months and was leapt upon on Tuesday by the Department for Work and Pensions, which said it would hold an inquiry into why the numbers are different.

In fact, differences between the two in the shorter term are quite normal even if they tell a similar story over the longer term. The two measure different things over different time periods so differences are not unusual.

The claimant count, as its name suggests, counts the number of people out of work and claiming benefit. Thus it is a fairly narrow measure and shows unemployment currently at 1.6 million, or 4.9% of the workforce, although those figures are the highest since 1997.

The LFS measure, also known as the International Labour Organisation (ILO) count, is based on surveys of large numbers of families and so picks up those out of work and not claiming benefits, for whatever reason. Under that measure, unemployment is nearer 2.5 million, or 7.8% of the workforce.

The claimant count is carried out on a monthly basis whereas the LFS is a rolling, three-monthly number lagging a month behind the latest claimant count figure. So yesterday's claimant count figures were for July while the LFS was for the April to June period.

Over the past year, both have risen by more than 700,000, so are telling roughly the same story.

In the past couple of months, though, increases in the claimant count have slowed sharply to 25,000 a month, while the LFS measure showed a rise of 220,000 in the latest three months, down from the record 281,000 increase seen in the three months to May.

Researchers at the Institute for Fiscal Studies have created a rolling, three-monthly series for the claimant count and compared the two. They say there have been recent periods when the ILO measure outpaces the claimant count for a time and then the claimant count catches up.

"That really only leads us to expect the claimant count to play 'catch up' again in the autumn, as some of these newly unemployed people start claiming benefits – though both series will hopefully slow as the recession eases," say Alastair Muriel and Luke Sibieta.

And single-month LFS figures from the Office for National Statistics show that in May and June unemployment rose more rapidly than in April, with June showing a jobless rate of 8.2% compared with 7.8% for the three months to June. That suggests that some economists' expectations that the slowing numbers of the claimant count would feed through into smaller LFS rises are almost certainly misplaced.

Philip Shaw, economist at Investec bank, agrees: "Overall we suspect that the ILO numbers are telling the more accurate story and that unemployment is still increasing relatively rapidly, although it is possible that the number of claimants is being stemmed by individuals accepting part-time work and topping up incomes with tax credits."

Another sign of distress in the labour market comes from the employment, rather than the unemployment, data. It shows a drop of 271,000 in the second quarter of the year. That is the biggest fall since records began in 1971. Given that the 1950s and 1960s had full employment, it could well be the biggest fall since the Great Depression.